Cable testing system and method



June'26, 1951 J. D. sow 2,553,435

, CABLE TESTING SYSTEM AND METHOD Filed Feb. 2'7, 1945 9 Sheets-Sheet '1 INVENTOR.

ATTOR JAMES D. 60w.

June 26, 1951 J. D. GOW 2,558,485

CABLE TESTING SYSTEM AND METHOD Filed Feb. 27, 1945 9 Sheets-Sheet 2 Fig.

Fig. 10

Fi .6 Fi .7 Fig.8

, INVENTOR. JAMES D. Gow

ATTORNEY.

June 26, 1951 J. D. GOW

CABLE TESTING SYSTEM AND METHOD 9 Sheets-Sheet 3 Filed Feb. 27, 1945 TNVENTOR. JAMES D. Gow

ATTORNEY J. D. GOW

CABLE TESTING SYSTEM AND METHOD June 26, 1951 9 snets-sheet 4 Filed Feb. 27, 1945 9 Sheets-Sheet 6 J. D. GOW

CABLE TESTING SYSTEM AND METHOD June 26, 1951 Filed Feb. 27, 1945 mvzurox JAM as D. 60w

ATTORNEY.

J. D. sow 2,558,485

CABLE TESTING SYSTEM AND METHOD June 26, 1951 9 Sheets-Sheet 8 Filed Feb. 27. 1945 Wk mm INVENTOR. JAMES D. Gow BY m EE ATTORNEY.

\ June 26, 1951 9 Sheets-Sheet 9 Filed Feb. 27; 1945 r| 1 W h 1 m .w y m w PM?!" N l w w w w w A A A J. J/ L m m M M m m w m mu 9 5. 9 9 9 m w H m m H m h F F Fatente .Fune 26, i951 CABLE TESTING SYSTEM AND METHOD Application February 27, 1945, Serial No. 580,046

44 Claims. (Cl. 175-183) The present invention relates to the testing of cables and the like and more particularly to improved methods and apparatus for determining the relative position of conductors in opaque cables.

An object of the inventionis to provide an improved method for determining the position of a conductor or other filamentary element in an elongated opaque body in which it is supported.

Another object of the invention is to provide apparatus for determining the position of a conductor or other filamentary element in an opaque body relative to a predetermined axis therein.

Another object of the invention is to provide an improved apparatus for determining the eccentricity of a coaxial cable, both as to the distance between the axes of the outer and inner conductors and as to the direction of the displacement between these axes.

Another object of the invention is to provide an arrangement for visually indicating the position of the inner conductor of a coaxial cable with respect to the outer conductor.

Another object of the invention is to provide a system for measuring the eccentricity of the inner conductor of a coaxial cable with respect to the cable sheath including an eccentricity testing device in which the accuracy of measuring the eccentricity does not depend on accurately centering the coaxial cable in the testing device.

Another object of the invention is to provide a testing device for determining the eccentricity of any desired section of a coaxial cable without the necessity of feeding the cable into the testing device from the end of the cable.

Another object of the invention is to provide a testing device adapted to measure and indicate the position of an inner conductor of a coaxial cable with respect to the cable sheath with a feeding mechanism for automatically feeding the cable therethrough to facilitate determining variations insuch position along the length of the cable.

Another object of the invention is to provide apparatus for automatically feeding a coaxial cable through an eccentricity testing device and for indicating the sections of the cable in which the position of the inner conductor with respect to the cable sheath lies outside of a predetermined range.

Another object of the invention is to provide in apparatus for automatically feeding a coaxial cable through an eccentricity testing device, an arrangement for arresting the automatic feeding of the cable through the testing device when a section of the cable in which an inner conductor has an eccentricity outside of a predetermined range with respect to the cable sheath enters the testing region.

Another object of the invention is to provide apparatus for facilitating the marking of different sections of a coaxial cable in accordance with the eccentricity of the inner conductor thereof with respect to the cable sheath in those sections or in accordance with some mechanical or electrical property of the cable dependent upon such eccentricity and afiecting the manner or the range of use of the cable.

The invention together with the foregoing and other objects and advantages thereof may be more readilyunderstood by reference to the following description taken in conjunction with the accompanying drawings, wherein Figure 1 is a diagrammatic end view of a testing device embodying features of the present invention, and including a rotating magnetic field generator;

Fig. 2 is a diagrammatic sectional view taken on the line 2-2 of Fig. 1 illustrating one system using the magnetic field generator in practicing the invention;

Fig. 3 is a diagrammatic sectional view, similar to Fig. 2, illustrating another system using the magnetic field generator in practicing the invention;'

Fig. 4 is a diagrammatic illustration of an em bodiment of the invention suitable for successivelytesting a series of sections of a cable; v

Fig. 5 is a schematic wiring diagram illustrating an embodiment of the invention including a control network for presenting a cross sectional view of a coaxial cable on the screen of a cathode ray tube;

Fig.6 is a schematic wiring diagram of the impulse generator of the control network of Fig. 5;

Fig. 7 is a schematic wiring diagram of the multivibrator of the control network of Fig. 5;

Fig. 8 is a schematic wiring diagram of the cathode ray intensity control circuitof the control network of Fig. 5;

Referring to Figs. 1 and 2, there is illustrated one form of cable testing device IQ for measuring the degree of eccentricity of a coaxial cable. This device |9 includes a, rotating magnetic field generator 28 having a circular magnetic yoke 24 provided with three poles 25, 26', and 21 extending radially inward toward the axis of the yoke 24 and having three field coils 28, 29, and 38 positioned on the three poles 25, 26, and 21 respectively. The three field coils 28, 29, and 33 are star-connected to a, three phase source of alternating current power, the inner ends of the three coils being connected together and the outer ends of the respective coils 28, 29, and 38 being connected through corresponding current controlling resistors 3|, 32, and 33 to the threeconductors 35, 36, and 31 of a three phase power line. The yoke 24 and the three-poles 25, 26, and 21 are symmetrically arranged about the central axis X of the magnetic field generator 25, the center of the yoke 24 coinciding with the central axis X and the poles 25, 26, and 21 being uniformly spaced about the inner circumference of the yoke 24. The inner tips 38, 39, and 40 of the poles 25, 26, and 21 define a test region in which a cable 22, comprising an outer conductor, or metallic sheath, 2|, an inner conductor 23, and insulation 231;, to be tested is placed. These pole tips 38, 39, and 48 are so arranged that when current is supplied to the field coils 28, 29, and 38 from diiierent phases of the three phase power source, a rotating magnetic field is produced in the test region between the pole tips.

The circular yoke 24 rests on a sheet 43 of somewhat compressible material on a base member 44 and is clamped securely to the base member 44 by means of bolts 49 cooperating with horizontally extending arms 45 and 48 adjacent the lower end of two brackets 41, 41. The two brackets 41, 41 carry two corresponding upright arms 48, 48 extending over opposite ends of the magnetic field generator 20, the two arms 48, 48 being provided with two corresponding circular openings 5| therethrough in coaxial alignment with the central axis X of the magnetic field generator 20. Two similar flanged tubular guide members 53, 53 of the same size and shape extend through the openings 5| in the upright arms 48, 48 and are rigidly secured thereto by means of bolts 55, the central openings in the guide members being in coaxial alignment with the central axis X of the magnetic field generator 20. The tubular guide members 53 serve to support the coaxial cable 22 in the test region between the tips 38, 39, and 48 of the salient poles 25, 26, and 21 with the axis of the sheath 2| of the coaxial cable 22 substantially in coincidence with the axis X of rotation of the magnetic field. Analternating current voltmeter 56 is connected across opposite ends of the inner conductor 23 of the coaxial cable 22. A phasemeter 51 is provided having two sets of input terminals, one set being connected across the field coil 28 and the other set being connected across the voltmeter 56, that to opposite ends of the conductor 23. The phasemeter 51 indicates the phase angle difference between the alternating electromotive force across the coil 28 and the alternating electromotive force induced in the irmer conductor 23 of the coaxial cable 22.

In operation, when three phase alternating current is supplied to the field windings 28, 28, and 30, a rotating magnetic field is produced in the test region between the tips 38, 39, and 48 of the salient poles 25, 26, and 21. The magnetic 4 flux passes radially through the salient poles and circumferentially through the yoke 24 forming continuous paths through the test region, the poles 25, 26, and 21, and the yoke 24. The angle of rotation of the magnetic field with respect to a, vertical line varies in synchronism with the phase angle of the electromotive force across the field coil28. As the magnetic field rotates about the predetermined axis X of rotation at the center of the magnetic field generator 28, an electromotive force is induced along the length of the inner conductor 23 of the coaxial cable 22 in an amount bearing a direct relation to the distance between the axis of the inner conductor 23 and the axis X of. rotation. Thus, when the axis of the inner r conductor 23 coincides with the axis of the sheath sheath is indicated by the reading of the phasebe reduced in a number of ways.

meter 51.

The voltmeter 56 may be calibrated directly in eccentricity by successively positioning in the testing device l8 each of a series of standard coaxial cables of known eccentricity and measuring the corresponding electromotive forces induced along the length of the inner conductors thereof when subjected to the rotating magnetic field. Similarly, the phasemeter 51 may be calibrated directly in angular position by successively positioning in the test device each of a series of standard coaxial cables having inner conductors located in known directions from the cable axis and measuring the corresponding phase angle difference.

Effects due to harmonics arising because of irregularities in the rotating magnetic field may In the first place such efiects may be reduced by utilizing a voltmeter 56 and a phasemeter 51 of a type which selectively responds to signals of line frequency. This may be accomplished for example when the line frequency is 60 C. P. S. and the field is rotating at 3600 R. P. M. by utilizing a vacuum tube voltmeter 56 and a phasemeter 51 including band pass filters at their inputs which selectively pass voltages in a range including 60 C. P. S. Such efiects may also be reduced by utilizing as a field generator 28 a two pole phase wound stator having a suitably distributed winding of the type used in induction motors. For this purpose for example a three phase Selsyn stator has proved particularly adaptable. With either of these arrangements the value of the electromotive force induced in the inner conductor 23 of the coaxial cable 22 is substantially proportional to the distance of the inner conductor from the axis of the field generator and the angular reading of the phasemeter corresponds linearly with the angular position of the inner conductor in the test region.

In view of the foregoing it willbe understood that the position of any conductor with respect to a. predetermined axis of a body supporting the conductor may be determined by establishing a magnetic field which rotates about that axis and measuring the amplitude and phase of the resultant alternating electromotive force induced in the conductor.

Referring now to Figs. 1 and 3, there is illustrated another cable testing device 59 for measuring the degree of eccentricity of a coaxial cable. The testing device 59 illustrated here is similar to that hereinabove described in connection with Figs. 1 and 2 and comprises the same rotating magnetic field generator 20. In this case also the yoke 24 rests on a sheet 43 of somewhat compressible material on a base member 44, but in this case is clamped securely to the base member 44 by means of two small brackets 60, 60 rigidly secured to the base by means of bolts 62. In this case no special precautions are taken to position the axis of the coaxial cable 22 in coincidence with the axis of rotation of the magnetic field and the coaxial cable 22 to be tested is supported in the test region between the pole tips 38, 39, and 40 with the axis of the coaxial cable 22 disposed substantially parallel to the axis of rotation of the magnetic field, simply by resting the cable 22 on the tip 38 of the lower magnetic pole 25. A short wire 63 electrically connects the outer and inner conductors 2| and 23 together at one end of the coaxial cable 22 and an alternating current voltmeter 64 is connected between the ends of the outer and inner conductors 2| and 23 at the other end of the coaxial cable 22. A phasemeter 65 is provided having two sets of input terminals, one set being connected across the field coil 28 and the other set being connected across the voltmeter 64.

In operation, when three phase alternating current is supplied to the field windings 28, 29, and 30, a magnetic field is produced in the test region between the tips of the salient poles, the magnetic field rotating about a predetermined axis in the manner hereinabove explained. As the magnetic field rotates, an electromotive force is induced along the length of the outer conductor 2| in an amount corresponding to the distance between the axis of the outer conductor 2| and the predetermined axis of rotation, and another electromotive force is induced along the length of the inner conductor 23 in an amount corresponding to the distance between the axis of the inner conductor 23 and the predetermined axis of rotation. Thus, when the axis of the inner conductor 23 coincides with the axis of the outer conductor 2| of the coaxial cable 22, the eccentricity of the coaxial cable 22 is zero and the two electromotive forces induced in the outer and inner conductors 2| and 23, respectively, are equal so that no electromotive force is indicated by the meter 64. However, when the axis of the inner conductor 23 does not coincide with the axis of the outer conductor 2| of the coaxial cable 22, the two alternating electromotive forces induced in the outer and inner conductors 2| and 23, respectively, of the coaxial cable 22 differ by an amount bearing a direct relation to the distance between the axes of the outer and inner conductors 2| and 23, respectively, and thus to the eccentricity of the inner conductor 23 with respect to the cable sheath 2|. The phase difference between the alternating electromotive force across the coil 28 and the alternating electromotive force induced in the inner conductor 23 indicated by the phasemeter 65 is a measure of the angular position of the inner conductor 23 with respect to the vertical line passing through the axis of the coaxial cable 22. The voltmeter 64 and the phasemeter 65 may be accurately calibrated by placing each of a series outer and inner conductors 2| and 23, the accuracy of measuring the eccentricity of a coaxial cable with this arrangement is not dependent upon centering the coaxial cable 22 in the magnetic field. This arrangement for measuring the eccentricity of a coaxial cable 22 is also less subject to effects caused by irregularities in the magnetic field. Such efiects may be reduced if de sired in the manner hereinbefore explained.

In view of the foregoing, it will be understood that the relative positions of any two substantially parallel conductors may be determined by establishing a magnetic field which rotates about an axis substantially parallel to the conductors and measuring the amplitude and phase of the resultant diflerence between the alternating electromotive forces induced in the conductors.

Referring now to Fig. 4, there is illustrated a. form of cable testing apparatus for measuring variations in the degree of eccentricity of a coaxial cable 12 having outer and inner conductors II and 13 from section to section along its length. This apparatus comprises a testing device 69 including a rotating magnetic field generator 10 and an arrangement of roller guides 90, 9|, 92, and 93 for positioning a section of the coaxial cable 12 to be tested in the rotating magnetic field with the axis of the coaxial cable 12 disposed along a line substantially parallel to the leaves 83 and 84 of a hinge 85 at one end thereof and normally locked together at the other end thereof by means of a pin 86 on the first segment 3| and a hook 81 pivotally supported on the second segment 82. The first segment 8| extends over a relatively small arc of about and supports one of the poles l6 and the second segment 82 extends over a relatively large arc of about 240 and supports the other poles l5 and 11. When locked in their normal, closed position, the two arcuate segments 8| and 82 constitute a circular yoke interconnecting the poles T5, 16, and 11 providing a closed magnetic circuit at the outer ends thereof. When the two arcuate segments 8| and 82 are unlocked and swung apart to an open position, a section of the cable 12 may be introduced into the test region defined by the tips of the poles 75, 16, and I! at the center of the magnetic field generator. In operation, the segments 8| and B2 are in their locked position so that when three phase alternating current is suppliei to the field coils 1B, 19, and 80, a rotatin magnetic field is established in the test region between the tips of the poles l5, l6, and 11, the magnetic field rotating about the axis of the magnetic field generator H1 in the manner hereinabove explained. A first pair of horizontal roller guides 90 and a first pair of vertical roller guides SI are arranged on one side of the magnetic field generator 10, and a second pair of hor zontal roller guides 92 and a second pair of vertical roller guides 93 are arranged on the other side of the magnetic field generator 10 to facilitate feeding the coaxial cable 12 through the test region between the poles 15, 16, and 11 along a line substantially parallel to the axis of rotation of the magnetic field.

A cable feeding mechanism is provided including a feed, or pay-out reel 94, and a take-up reel 95. The pay-out reel 94 is mounted on an axle 96 supported at opposite ends by two bearings 91 and 98. A friction brake 99 is mounted on the pay-out reel axle 96. The take-up reel 95 is mounted on an axle I supported on opposite sides of the take-up reel 95 by means of bearings WI and I02. Two insulated slip rings I03 and I04 are mounted on the take-up reel axle I00. Attached to one end of the take-up reel axle I00 is a three phase induction motor I05 preferably of the gear head speed reducing type adapted when suitably energized from the buses I06, I01, and I08 to drive the take-up reel 95 at a suitable testing speed. The two ends of the conductors H and 13 of the coaxial cable 12 on the pay-out reel 94 are-electrically connected together in any suitable manner and the two ends of the conductors H and 13 of the coaxial cable 12 on the take-up reel 95 are electrically connected by suitable means to the two slip rings I03 and I04. Two brushes I09a and M92) contacting the slip rings I03 and I04, respectively, are connected to the input of a voltage amplifier I I I.

An adjustable oil-normal voltmeter I I0 is connected in the output of the voltage amplifier I I I and includes an indicator hand I I0a for indicating the voltage appearing in the output of the amplifier III. The off-normal voltmeter III] also includes normally open contacts II2 which are closed when the voltage in the output of the amplifier I II exceeds a predetermined value. A power relay H4 is provided having a winding I2I and normally open main contacts H5 and normally open sticking contacts I23. The main contacts are included between the buses I06, I01, and I08 and the driving motor I05. A normally open start button I20 is included in series with the winding I2I across the two buses I06 and I01 and a normally closed stop button I22 is included in series with the normally open sticking contacts I23 across the start button I20. An off-normal relay I25 is also provided having a winding I26 included in series with the off-normal contacts II2 of the oil-normal voltmeter IIO across the two buses I06 and I01. The off-normal relay I25 includes a pair of normally closed contacts I 21 in series with a normal pilot light I 28 across the two buses I06 and I01 and also includes a pair of normally open contacts I29 in series with an oil-normal pilot light I30 across the two buses I06 and I01. A bell I3I is connected in parallel with the off-normal pilot light I30. The off-normal relay I25 also includes a pair of normally closed contacts I32 in series with the stop button I22 and the sticking contacts I23 of the power relay I I4.

To determine variations in eccentricity of a coaxial cable 12 along its length, the spool on which the coaxial cable is wound is positioned in the cable feeding mechanism as a pay-out reel 94 with the ends of the outer and inner conductors H and 13 of the cable 12 on the innermost winding thereof on the pay-out reel electrically connected together. The opposite end of the cable 12 is threaded onto the take-up reel with the ends of the outer and inner conductors TI and 13 of the coaxial cable 12 respectively connected to the two slip rings I03 and I04. The two segments 8| and 82 of the yoke 14 are unlocked and the yoke 14 opened and the cable 12 introduced through the opening in the yoke 14 into the test region. The two segments 8| and 82 of the yoke 14 are then looked together in the manner hereinabove set forth. The guide rollers 90, 9|, 92, and 93 are then positioned about the cable 12 in a manner suitable for supporting the cable 12 in the test region between the poles 15, 16, and 11 with the axis of the cable 12 substantially parallel to the axis of the magnetic field generator 10.

In order to initiate operation of the cable testing apparatus, the circuit breaker I I3 is closed connecting the buses I06, I01, and I08 to a source of three phase power, and thus causing a rotating magnetic field to be established in the test region at the center of the magnetic field generator 10 in the manner hereinabove explained. As the magnetic field rotates, electromotive forces are induced along the outer and inner conductors TI and 13, respectively, of the coaxial cable 12. The difierence between the electromotive forces induced along the outer and inner conductors H and 13 is communicated through the slip rings I03 and I 04 and the brushes I09a and I09b to the input of the amplifier III and the difference in electromotive force amplified and impressed upon the voltmeter IIO. Accordingly, the position of the indicator hand 00. of the voltmeter H0 indicates an electromotive force corresponding to the eccentricity of the section of the coaxial cable 12 located in the test region. Also, when the circuit breaker H3 is closed, the circuit including the normal" pilot light I28 is completed through the normally closed contacts I21 of the off-normal relay I25. If the eccentricity of the section of the cable 12 in the test region is less than a predetermined value, the normally open contacts II2 of the meter IIO remain open and the normal pilot light I 28 remains lit. However, if the eccentricity of the section of the cable 12 in the test region is greamr than the aforementioned predetermined value, the normally open contacts II2 of the meter IIO are closed and the winding I25 of the oiT-normal relay I 25 is energized. When the winding I26 of the offnormal relay is thus energized, the normally closed contacts I 21 are opened thus de-energizing the "normal" pilot light I28 and the normally open contacts I29 are closed thus energizing the ofi-normal pilot light I30 and the bell I3I.

Considering now the operation of the feeding mechanism when the eccentricity of the section of the cable 12 initially positioned in the test region has a value less than the aforementioned predetermined value, the feeding of the cable through the test region is initiated by momentarily depressing the start button I20. When the start button I20 is depressed, the winding I2I of the power relay H4 is connected across the two buses I06 and I01, thus energizing the power relay II4 and closing the normally open main contacts II5, thereby connecting the driving motor I05 to the buses I06, I01, and I58 which have been previously energized. Simultaneously, the normally open sticking contacts I23 are closed, thereby completing a circuit through the winding I2I of the power relay II4 through the normally closed contacts I32 of the off-normal relay when the start button I20 is subsequently released thereby retaining the power relay II4 in its operated condition; When connected to the source of three phase power, the driving motor I05 operates the cable feeding mechanism causing the take-up reel 95 to rotate at a suitable preestablished testing speed and drawing the cable I2 from the pay-out reel 94 through the test region onto the take-up reel 95. While being drawn through thetest region, the cable 12 is maintained in a substantially fixed predetermined position therein by means of the guide rollers 90, 9|, 92, and v93. By virtue of the action of the friction brake 99, the cable I2 remains taut while being drawn through the test region.

As the cable 12 is drawn through the test region,

" the difference between the electromotive forces induced along the outer and. inner conductors 'II and I3, respectively, in the sections of the cable I2 successively fed into the test region varies in accordance with the eccentricity of the cable I2 in these sections, and the changes in eccentricity of the cable from section to section are indicated by the pointer I Ia of the voltmeter IIO. As long as the eccentricity of the sections of cable subjected to test in the test region remains below the aforementioned pre determined value, the normally open off-normal contacts II2 of the voltmeter IIO remain open, and the normal pilot light I28 remains energized. However, as soon as a section of the cable I2 having an eccentricity greater than the aforementioned predetermined value enters the test region, the off-normal contacts II2 of the voltmeter III) are closed, causing the winding I26 of the off-normal relay I25 to become energized. When the winding of the off-normal relay I25 is thus energized, the normally closed contacts I21 are opened and the normally open contacts I29 are closed, thus de-energizing the normal pilot light I 20 and energizing the off-normal pilot light I30 and energizing the bell I3 I. Simultane ously, upon energization of the winding I25 of the off-normal relay I25, the normally closed contacts I32 of the ofi-normal relay I25 are opened, thereby opening the circuit including the power relay winding I2I and the stop switch I22 and sticking contacts I23, thus de-energizing the power relay winding I2 I. When the winding I2I is thus de-energized, the power relay H4 is restored to its normal position and the main contacts II5 thereof are opened de-energizingthe driving motor I05 and accordingly arresting the operation of the feeding mechanism which thereupon stops quickly by virtue of the resistance offered by the brake 99 on the pay-out reel axle 96. When operation of the feeding mechanism is thus arrested, the section of the cable of ab normal eccentricity is marked on the exterior surfaces of the cable with a legend indicating the eccentricity in any suitable manner.

In order to determine the extent of the portion of the cable of abnormal eccentricity, the start button I20 is depressed and held in its operated position thereby energizing the winding I2I of the power relay H4 and causing the cable 12 to be continuously fed through the testing mechanism. When a section of the cable of normal eccentricity next enters the testing device 69, the contacts II2 of the off-normal 10 voltmeter I I0 are opened, thus de-energizing the winding I26 of the off-normal relay I25. When the winding I 26 of 'the off-normal relay I is thus de-energized, the contacts I29 are opened and the contacts I21 closed. Thus, when the winding I26. of the off-normal relay I25 is deenergized, the off-normal" pilot light I30 and the bell I3I are de-energized, and the "normal pilot light I 28 is energized. Simultaneously, upon de-energization of the winding I26 of the oilnormal relay I25, the closing ofthe contacts I32 completes the circuit including the sticking contacts I23 and the winding I2I 0f the power relay I I4, thus permitting the start switch I20 to be released without de-energizing the Winding I2I of the power relay II4. With the winding I2I of the power relay I I4 thus energized again through the circuit including the sticking contacts I23, the automatic operation of the. cable feeding mechanism is resumed.

When sections of cable 12 of normal eccentricity are being fed throughthe testing device 59 in the manner hereinabove described, the operation of the cable feeding mechanism may be arrested at any time by depressing the button of the normally closed stop switch I22. When this button is depressed, the stop switch I22 is opened thus disconnecting the winding I2I of the power relay II4 from the two buses I05 and I01. The resultant de-energization of the winding I2I causes the power relay I I4 to restore to its normal position thereby opening the main contacts H5 and thereby disconnecting the driving motor I05 from the source of alternating current power.

As the cable is being fed through the testing region, various sections of the cable are marked on the exterior surface thereof ,with legends denoting values of eccentricity as" indicated by the meter I I0, or as indicated by theofi-normal relay I25. Ifdesired, instead of marking the cable with legends in terms of eccentricity at the various sections thereof, the cable may be marked with legends in terms of some other characteristic of the cable related to the eccentricity. Such characteristics include, for example, the safe operating voltage that may be applied between the conductors of the coaxial cable at the various sections thereof, or the impedance, or

reflection coefficient, or attenuation constant,

offered by the coaxial cable to radio frequency waves propagated through the annular space between the 'outer and inner conductors thereof.

In view of the foregoing, it will be understood that the variations in relative positions of any two conductors of a cable along the length thereof may be determined by feeding the cable through a rotating magnetic field along a line more or less parallel to the axis of rotation and measuring the changes occurring in the resultant difference be tween the alternating electromotive forces induced in the conductors.

Referring now to Fig. 5, there is illustrated another form of cable testing apparatus embodying features of the present invention adapted to present on a viewing screen of a cathode ray tube an actual cross sectional picture of the section of a coaxial cable undergoing test. This cable testing apparatus comprises a testing device I49 including a magnetic field generator I50 for establishing a rotating magnetic field which is substantially constant and which is substantially homogeneous over a test region in which a section tube 200 controlled jointly in accordance with the intensity of the rotating magnetic field and in accordance with the difference in the'electromo tive forces respectively induced along the lengths of the outer and inner conductors II and I53 of the coaxial cable I52 for indicating on the oathode ray tube viewing screen 208 the actual position of the inner conductor I53 with respect to the outer conductor I5I of the coaxial cable I52.

More particularly, the magnetic field generator I50 comprises first, second, and third pairs of sistors I11 and I19 and also the side of the input I511), respectively, and first, second, and third field windings I58, I59, and I60, respectively associated with said pole pairs, and a circular yoke I54, the poles I55a, I55b, I56a, I56b, I51a, and I51b extending radially inward therefrom. The first pair of poles I55aI55b are disposed along a vertical line and the second pair of poles I560- I56b are inclined 60 in'one direction from the vertical and the third pair of poles I51aI51b are inclined 60 in the opposite direction from the vertical, the six poles being equally spaced circumferentially within the circular yoke I54. The three field windings I58, I59, and I60 are starconnected to a three phase power line comprising first, second, and third conductors I6I, I62, and I63 respectively connected to a source of three phase power. Each of the windings I58, I59, and

I60 is equally divided between opposite poles of the corresponding pair of poles, the entire arrangement constituting, in effect, a three phase six pole stator. When equal three phase alternating currents are supplied to the field windings I58, I59, and I60 of such a stator of suitable design, a substantially homogeneous magnetic field of substantially constant intensity is established in a test region defined by the tips of the six poles I55a, I55b, I56a, I56b, I51a, and I51b, this magnetic field rotating at a substantially uniform rate about the central axis of the magnetic field generator I50. For purposes of explanation, it is here assumed that the phases of the alternating currents supplied to the three field windings I58, I59, and I60 are so related that the magnetic field generated in the test region rotates in a counterclockwise direction about the central axis of the magnetic field generator I50, as indicated by the arrow I65, and that when the instantaneous value of the alternating current through the field coil I58 is a maximum, the magnetic field in the test region is vertical.

A step-up transformer I10 is provided having its primary winding I1I connected across the second and third power line conductors I62 and I63, and having its secondary winding I12 connected across the input I13 of a first phase transformation network I14. The first phas'e transformation network I14 has first and second output terminals I15 and I16, respectively, and is of such design that the single phase alternating current voltage applied to its input I13 is transformed into two voltages at the output terminals I15 and I16 which are 90 out of phase, the two voltages referred to appearing between the respective output terminals I15 and I16 and ground. More particularly, the first phase transformation network I14 includes a first resistor I11 and a first condenser I18 connected in series across the input I13, and a second resistor I19 and a second condenser I80 connected in series across the first condenser I18, the second resistor I19 being connected to the junction between the first resistor I11 and the first condenser I18. The ends of the two condensers I18 and I80 remote from the reall] I13 to which they are directly connected, are grounded. The first terminal I15 is connected to the ungrounded side of the input I13, that is at a point between the resistor I11 and the adjacent end of the secondary winding I12 of the transformer I10. The second output terminal I16 is connected to the junction between the second resistor I19 and the second condenser I80. .The constants of the resistors I11 and I19 and the condensers I18 and I are so proportioned that the output voltage at the second output terminal I16 leads the output voltage at the first output terminal I15 by 90.

Adjacent ends of the outer and inner conductors I5I and I53 at one end of the coaxial cable I52 are electrically connected together while the ends of the outer and inner conductors I5I and I53 at the other end of the coaxial cable I52 are connected to the input I82 of voltage amplifier I83, the outer conductor I5I being connected to 'the grounded side of the input I82. The output I84 of the amplifier I83 is connected through an adjustable phase shifter I85 to the input I86 of a second phase transformation network I81 similar to the first phase transformation network I14 hereinabove described. The second phase transformation network I81 has first and second output terminals I88 and I89, respectively, and is also of such design that the single phase alternating current voltage applied to its input I86 is transformed into two voltages at the output terminals I88 and I89 which are 90 out of phase, the two voltages referred to here likewise appearing between the respective output terminals I88 and I89 and ground. More particularly, the second phase transformation network I81 includes a first resistor I90 and a first condenser I9I connected in series across the input I86 and a second resistor I92 and a-second condenser I93 connected in series across the first condenser I9I, the second resistor I92 being connected to the junction between the first resistor I90 and the first condenser I9I. The ends of the two condensers I9I and I93 remote from the resistances I90 and I92 and the side of the input I86 to which they are directly connected, are grounded. The first output terminal I88 is connected to the other side of the input I86, that is, at a point between the resistor I98 and the output of the phase shifter I85. The second output terminal I89 is connected to the junction between the second resistor I92 and the second condenser I93. The constants of resistors I90 and I92 and the condensers I9I and I93 are so proportioned that the output voltage at the second output terminal I89 leads the output voltage at the first output terminal I88 by 90.

The cathode ray tube 200 has an electron emissive cathode 20 I an electron beam intensity control grid 202, an electron accelerating anode 203, upper and lower vertical beam deflection plates 224 and 205, respectively, left and right horizontal beam deflection plates 206 and 201, respectively, and a fluorescent anode 208 in the form of a viewing screen. The accelerating anode 203 is grounded; the electron emissive cathode MI is maintained at a high negative potential by means of a battery 2 I 0 connected between the accelerating anode 203 and the cathode 20I; and the beam intensity control grid 202 is maintained negative with respect to the cathode 20I by means of a bias battery 2| I. A resistor 2I2 is connected between the bias battery 2| I and the electron emissive cathode 20I for impressing suitable electron beam intensity control voltages on the control complished by operating the multivibrator 210 at through a maximum in the opposite direction at grid 202 in a manner more fully set forth hereinbelow. The horizontal deflection plates 205 and 201 of the cathode ray tube 200 are connected across the output 2I5 of a first balanced amplifler 24 6. Similarly, the vertical deflection plates 204 and 205 of the cathode ray tube 200 are connected across the output 2" of a second balanced amplifier 2I8.

A control network 2I9 is provided which operates to alternately impress the output voltages of lo the respective phase transformation networks I14 and I81 periodically on the inputs 220 and MI of the balanced amplifiers 2I6 and 2I8. The control network 2I9 also operates to impress a control voltage periodically on the beam intensity control grid 202 of the cathode ray tube 200. The control voltage and the output voltages are so coordinated that the control network 2I9 operates to produce an image pattern on the screen 200 of the cathode ray tube 200 which represents the device I49, as more 'fully explained hereinbelow.

More particularly, the control network 2I9 includes an impulse generator 230, an asymmetrical multivibrator 210, an intensity control circuit 290, and an electronic switch 320. The impulse generator 230, the multivibrator 210, and the electronic switch 320 cooperate to alternately a0 transfer voltages from the output terminals I15 and I 15 of the first phase transformation network I14 and from the output terminals I88 and I89 of the second phase transformation network I81 periodically through the balanced amplifiers 2I6 and 2 I8 to the beam deflection control plates 204. 205, 206, and 201 of the cathode'ray tube 200. The impulse generator 230 and the multivibrator 210 cooperate to produce a periodically variablecontrol voltage in the intensity control circuit 290 which is used to control the intensity of the electron beam projected onto the screen 208 of the cathode ray tube 200. The operation of this cable testing apparatus isfacilitated by operating the control network 2I9 periodically at a frequency half that supplied from the power source to the magnetic field'generator I50. This is achalf the power line frequency. Considering in a general way a cycle of operation beginning at an instant when the rotating magnetic field produced by the magnetic field generator I is vertical, it will be assumed for simplicity of explanation that the power line frequency is 60 cycles per second and that the cone trol network 2I9 operates at 30 cycles per second. For convenience, a cycle of operation is divided into four quarters as indicated in Figs. 11a to llq, the first quarter beginning at time To and ending at time T1; the second quarter beginning at time T1 and ending at time T2; the third quarter beginning at time T2 and ending at time T3; and the fourth quarter beginning at time T3 and ending at time T4. For convenience, a cycle of operation is also divided into first and second operating portions, the first operating portion overlapping the first and second quarters and the second portion being encompassed within the third and fourth quarters as more fully set forth hereinbelow. The current through the field winding I58 on the vertical poles Ia and I55b of the magnetic field generator I50 varies sinusoidally at 60 cycles per second, passing through a maximum in one direction at times T0, T2, and T4 and times T1-and Ta, as illustrated in Fig. 11a. The magnetic field rotates in a counterclockwise direction. the angular position of the magnetic field relative to a vertical axis changing from 0 to 180 and then to 360 at a uniform rate twice in each cycle of operation, as illustrated in Fig. 11b. More specifically, the magnetic field makes an angle of 0 with the vertical at times T0, T2, and,

T4 thus pointingvertically upward at these times, and makes an agle of 180 with the vertical at times T1 and Ta, thus pointing vertically downward at thesetimes. The voltage induced between the outer and inner conductors I5I and I53 of the coaxial cable reaches a maximum when the rotating magnetic field extends along a line through the axes of the conductors I5I and I53.

Thus, for example, with the inner conductor I53 positioned 30 counterclockwise from a vertical line passing through the axis of the outer conductor I 5|, the voltage induced between the outer and inner conductors I5I and I53 of the coaxial cable I52 lags the current through the field coil I58 by a time phase angle of 30 reaching a maximum value in one direction V1 sec. after times T0, T2, and T4 and reaching a maximum value in the other direction see. after times T1 and T: as illustrated in Fig. 11c.

Assume for simplicity of explanation that the current through the input I13 of the first phase transformation network I14 is in phase with the current through the field coil I58 and that the voltage at the first output terminal I15 of the phase transformation network I14 is likewise in phase with the current through the field coil I58, as illustrated in Fig. 11d. Then, the voltage at the second output terminal I16 of the phase transformation network I14 leads the current through the field coil I58 by 90, as illustrated in Fig. lle. In a similar manner, assuming for simplicity of explanation that no phase change occurs in the amplifier I83 and the phase shifter I85, the voltage at the first output terminal I88 of the second phase transformation network I81 is in phase with the voltage difference between the outer and inner conductors I5I and I53 of the coaxial cable I52, as illustrated in Fig. 11!. Likewise then, the voltage at the second output terminal I89 of the second phase transformation network I81 leads the voltage difference between the innerand outer conductors of the coaxial cable by 90, as illustrated in Fig. 119. It will be understood, of course, that the phase relations of the voltages at the output of the'second phase transformation network I81 relative to the voltage between the second and third conductors I62 and I63 to which the input I13 of the first phase transformation network I14 is connected may be varied either by adjustment of the phase shifter I or by rotary adjustment of the magnetic field generator I50 about its axis to establish the appropriate phase relations between the voltages at the output terminals of the two phase transformation networks I14 and I81.

Considering how the general arrangement and operation of the elements in the control system 2I9, the impulse generator 230 is provided with two terminals 23Ia and 23Ib at the input 23I thereof and first and second output terminals 232 and 233, respectively, at the output 234 thereof. The second and third conductors I62 and I63 are connected respectively to the input terminals 23 Ia and 23Ib of the impulse generator 230. The alternating current voltages appearing between these conductors I62 and I53 impressed on the input 23I of the impulse generator 230 are converted therein into sharp positive pulses which appear at the first output terminal 232 thereof and sharp negative pulses which appear at the second output terminal 233 thereof. The constants of the elements forming the impulse generator 230 are so selected that these sharp pulses appear at the output 234 at the same time that the magnetic field produced in the inductive field generator I50 is vertical; that is, at times To, Ti, T2, T3, and T4, as illustrated in Fig. 11):. The first operating portion of the cycle of operation includes the intervals during which the sharp pulses exist at times To, Ti, and T: and the second operating portion of the cycle of operation includes the remaining part of the cycle of operation.

The multivibrator 210 is provided with an in ut terminal 21l, a first output terminal 212, a second output terminal 213, and a third output terminal 214. The multivibrator 210 generates rectangular waves which appear at the output terminals 212, 213, and 214 thereof, as more fully set forth hereinbelow, the free period of the oscillations of the multivibrator being slightly greater than twice the first operating portion of the cycle of operation. The input terminal 21| of the multivibrator 210 is connected to the first output terminal 232 of the impulse generator 230. Positive pulses transmitted from the output 234 of the impulse generator 230 to the input terminal 21! of the multivibrator 210 cause the signals appearing at the output terminals 212, 213, and 214 to vary in synchronism with the rotation of the magnetic field in the magnetic field generator I50 forcing the oscillation of the multivibrator 210 to be asymmetrical. More particularly, the voltage appearing at the first output terminal 212 of the multivibrator 210 falls to a low positive value shortly before time To and remains at this value until shortly after time T2, then rises to a relatively high positive value where it remains until shortly before time T4, as illustrated in Fig. lli. And, more particularly, the voltage appearing at the second output terminals 213 of the multivibrator 210, and also the voltage appearing at the third'output terminal 214 of the multivibrator 210 rises to a relatively high positive value shortly before time To and remains at that value until shortly after time T2, then falls to a relatively low positive value where it remains until a time shortly before time T4, as illustrated in Fig. 119'. Thus, the voltage at the first output terminal 212 is of a relatively low positive value and the voltages appearing at the second and third output terminals 213 and 214 are of a relatively high positive value during the first operating portion of the cycle, and the voltage at the first output terminal 212 is of a relatively high positive value and the voltages appearing at the second and third output terminals 213 and 214 are of a relatively low positive value during the second operating portion of the cycle.

The intensity control circuit 290 is provided with first and second input terminals 291 and 292, respectively, and an output terminal 293. The output terminal 293 is connected to the junction between the battery 2 and the resistor 2l2 between the cathode I and the grid 232 of the cathode ray tube 200. The first input terminal 29I of the intensity control circuit 290 is connected to the third output terminal 214 of the multivibrator 210. The second input terminal 292 of the intensity control circuit 290 is connected to the second output terminal 233 of the impulse generator 230. Rectangular voltage .III

16 waves appearing at the third output terminal 214 of the multivibrator 213 are thus impressed on the first input terminal 291 of the intensity control circuit 290 and negative pulses appearing at the second output terminal 233 of the impulse generator 230 are impressed on the second input terminal 292 of the intensity control circuit 233. The intensity control circuit 290 is so arranged that the voltage appearing at the output terminal 293 is ositive whenever the voltage impressed on the first input terminal 29l is of a relatively high value and whenever the voltage impressed on the second input terminal 292 is of a relatively low value and is negative at all other times. Thus the voltage appearing at the output terminal 293 of the intensity control circuit 290 rises to a positive value shortly before time To and remains at that value until shortly after time T2; then falls to a negative value where it remains until a time shortly before the time T3 when it rises again to a posltive value for a short interval, falling again to a negative value shortly after time T: where it remains until a time shortly before time T4, as illustrated in Fig. 11k. More particularly, the voltage at the output terminal 293 is positive during the first operating portion of the cycle of operation and for a short interval in the middle of the second operating portion and is negative in the remaining part of the second operating portion. When this control voltage is positive it reduces the bias on the control grid 21:2 of the cathode raytube 200 causing an intense beam of electrons to be projected through the accelerating anode 203 to the screen 208 and when this control voltage is negative it increases the bias on the control grid 232 cutting oil the electron beam entirely.

The electronic switch 328 is provided with first, second, third, and fourth input terminals 32!, 322, 323, and 324, respectively, first and second output terminals 325 and 326, respectively, and first and second control terminals 321 and 328, respectively. The first and second input terminals 32] and 322 are connected respectively to the second and first output terminals I16 and I15 of the first phase transformation network I14. Similarly, the third and fourth input terminals 323 and 324 are connected respectively to the first and second output terminals I88 and I89 of the second phase transformation network I81. The first and second output terminals 325 and 326 of the electronic switch 320 are connected respectively to the inputs 220 and 22| of the first and second balanced amplifiers 2l6 and M8. The first and second control terminals 321 and 328 of the electronic switch 320 are connected respectively to the first and second output terminals 212 and 213 of the multivibrator 210.

In operation, when the control voltage applied to the first control terminal 321 is at a relatively low positive value, that is, during the first portion of the cycle of operation, a connection is provided within the electronic switch 320 between the first input terminal 32! and the first output terminal 325 and another connection is provided within the electronic switch 320 between the second input terminal 322 and the second output terminal 323. Similarly. in operation when the control voltage applied to the second control terminal 328 is at a relatively low positive value, a connection is provided within the electronic switch 320 between the third input terminal 323 and the second output terminal 326 and another connection is provided within the electronic switch 320 between the fourth input terminal 324 ,at the second output terminal I16 of the first phase transformation network I14 is transmitted through the first balanced amplifier 2I6 to the f at the output of the second phase transformation and I53 of the coaxial cable I52. Likewise, the

horizontal deflection plates 206 and 201 of the cathode ray tube 200. The resultant electric fields between the plates are 90 out of phase and are so proportioned that they cause the electron beam projected from the cathode I through the accelerating anode 203 to the screen 208 of the cathode ray tube 200 to scan a first circular path 4! on the screen 208 having a radius substantially proportional to the amplitude of the electromotive force appearing between the two conductors I62 and I63 of the three phase power line and substantially proportional to the intensity of the rotating magnetic field produced by the generator I50. Similarly, in operation when the voltage at the second control terminal 328 is at a relatively low positive value, that is, during the second portion of the cycle of operation, the voltage appearing at the first output terminal I88 of the second phase transformation network I81 is transmitted through the second balanced amplifier M8 to the vertical deflectionplates 204 and 205 of the cathode ray tube 200, and the voltage appearing at the second output terminal I89 of the second phase transformation network I81 is transmitted through the first balanced amplifier 2I6 to the horizontal deflection As before, the resultant electric field's between the respective pairs of plates are 90 out of phase and in this case are so proportioned that they cause the electron beam projected from the oathode 20I through the accelerating anode 203 to the screen 208 of the cathode ray tube 200 to scan a second circular path (not shown) on the screen, concentric with the first circular path 4H] and having a radius proportional to the amplitude of the difl'erence in the electromotive forces induced along the lengths of the outer and inner conductors I5I and I53 of the coaxial cable I52 by the rotating magnetic field.

Considering now the general operation of the cable testing apparatus illustrated in Fig. 5, it will be noted that while the voltages appearing at the output of the first phase transformation network I14 are being transferred to the deflection plates 204, 205, 206, and 201 of the cathode ray tube 200 during the first operating portion of the cycle of operation, the bias on the intensity control grid 202 of the cathode ray tube 200 is at a relatively low value. Thus, when the voltages apmaring at the output of the first phase transformation network I14 are impressed on the deflection plates 204, 205, 206, and 201 of the cathode ray tube 200, a visible circular image M0 is produced on the screen. Similarly, it will be noted that while the'voltages appearing at the output of the second phase transformation network I81 are beingtransferred to the deflection plates 204, 2135, 205, and 201 of the cathoderay tube 200 during the second operating portion of direction of this spot 2 from the center of the circular image 4I0 corresponds to the direction of the axis of the inner conductor I53 from the axis of the outer conductor I5I of the coaxial cable I52. In practice, the circuit constants of the various elements of the magnetic field generator I50 and the control network 2I9 are so chosen that the radius of the circular image H0 is proportional to the intensity of the rotating magnetic field generatedby the field generator I50 irrespective of variations in the voltage 'supplied to the fleld coils I58, I59, and I60 from the three phase power line within the range of operation. More particularly, this is accomplished by operating the transformer I10 and the generator I50 over about the same portion of the magnetization curve of the iron used in each. Likewise, in practice, the circuit constants of the various elements of the control network 2I9 are so chosen that the position of the spot image 4I 2 within the circular image 4I0 corresponds geometrically to the position of the inner conductor I53 within the outer conductor I5I of the coaxial cable I52 irrespective of variations in the voltage supplied to the field coil windings I58, I59, and I 60 from the three phase power line. Under these conditions the image pattern formed on the screen 208 of the cathode ray tube 200 presents an actual cross sectional view of the section of the cable I52 undergoing test.

Referring now to Fig. 6, there is shown a detailed wiring diagram of the impulse generator 230. The impulse enerator 230 includes an input voltage step-up transformer 235 having opposite ends of its primary winding 236 connected between the input terminals 23Ia. and 23"). The

impulse generator 230 also includes a pair of triodes 231 and 238. The cathodes 239 and 240 of the triodes 231 and 238 are connected through a common parallel resistor-condenser network 24I to the grounded center tap 242 of the secondary winding 243 of the transformer 235. The anodes 245 and 246 of the triodes 231 and 238 are connected through a common load resistance 244 to the positive terminal of a voltage supply. And the control grids 241 and 248 respectively ,of the triodes 231 and 238 are connected to onposite ends of the secondary winding 243. Belatively high rid leak resistors 249 and 250 of the order of 100,000 ohms are connected betweenthe respective control grids 241 and 248 and the cor-' responding ends of the secondary winding 243.

' 21I of the multivibrator 210. The voltage apthe cycle of operation. thebias on the intensity pearing at the junction 25I is similarly impressed through a phase inverting network 253 to the second output terminal 233 of the impulse generator and thence to the second input terminal 292 of the intensity control circuit 290.

The coupling network 252 includes a blockaccuse 19 ing condenser 254, a 255 and a diode 256 connected in series. The anode 251 of the diode 256 is connected to the junction 25l through the resistor 255 and the condenser. 254, and the cathode 258 of the diode 256 is connected to the first output terminal 232 of the impulse generator 236. A by-pass resistor 259 is connected between the cathode 258 and anode 251 of the diode 256 in order to prevent accumulation of electrical charge on the cathode 258 and the anode 251. The coupling network 252 operates unilaterally to transmit positive pulses to the multivibrator 216 and to block the return of positive pulses from the multivibrator 216 to the junction 251 and thence to the phase inverting network 253. The phase inverting network 253 includes triode 256 connected as a degenerative amplifier having an input resistor 264 connected between the grid 26! and ground and a bias resistor 265 I connected between cathode 266 and ground. The control grid 261 of this triode 266 is connected through a coupling condenser 262 to the junction HI and the anode 263 of the triode 266 is connected to the second output terminal 233 of the impulse generator.

Considering now the cycle of operation of the impulse generator 236, when alternating current is impressed on the primary winding 236 of the transformer 235, relatively high alternating voltages are induced in the secondary 243 thereof and communicated through the grid leak resistors 249 and 256 to the control grids 241 and 248 of the respective triodes 231 and 238, the voltages impressed thereon being out of phase. When the voltage. impressed on the control grid 241 of the triode 231 is negative, very little current flows from the cathode 238 to the anode 245, but when this control grid 241 is driven positive, considerable current flows both to the rid 241 and to the anode 245. The current flowing to the grid 241 passes through the grid leak resistance 249 effectively maintaining the grid 241 at a substantially constant potential during the major portion of the positive quarters of the cycle of operation. As a result, the current flowing to the anode 245 reaches a relatively high, substantially constant value during the positive quarters of the cycle of operation. Thus the current flowing to the anode 245 attains a relatively low value throughout most of the first and third quarters of the cycle of operation and attains a relatively high value throughout most of the second and fourth quarters thereof, as illustrated in Fig. 111. In a similar manner, the current to the anode 246 of the triode 238 attains a relatively high value throughout most of the first and third quarters of the cycle of operation and attains a relatively low value throughout most of the second and fourth quarters of the cycle of operation, as illustrated in Fig. 11m. The total current through the common resistance 244 and the common resistor condenser network 24l remains at a substantially high value throughout the cycle of operation except at the times T0, T1, T2, T3, and T4 when it dips sharply for a very short time, as illustrated in Fig. lln. As a result, the voltage at the junction 25l between the resistance 244 and the two anodes 245 and 246 of the triodes 231 and 238 remains at a relatively low value throughout the cycle of operation except at the times T0, T1, T2, T3, and T4 when it rises sharply for a short time. Thus sharp voltage pulses are produced at the output 234 periodically at a frequency twice that of the input voltage, that is, at a frequency of 120 cycles current limiting resistor 20 per second, as previously illustrated in Fig. 11h. In view of the foregoing, it will be understood that the impulse generator supplies positive pulses to the multivibrator 216 and negative pulses to the intensity control circuit 296 at times To, T1,'Tz, T3, and T4 in the cycle of operation, that is at the times when the magnetic field being generated in the test region of the inductive field generator I56 is vertical.

Referring now to Fig. 7, there is shown a conventional' detailed wiring diagram of the multivibrator 216. This multivibrator 216 comprises a first triode 215 having a grounded cathode 218, a control grid 211, and an anode 218, and also comprises a second triode 286 having a grounded cathode 28l, a control grid 282, and an anode 283. The anodes 218 and 283 are connected to the positive terminal of a power supply through corresponding plate resistors 219 and 284. A first grid resistance 285 is connected between the grid 211 and cathode 216 of the first triode 215 and a second grid resistance 286 is connected between the grid 282 and the cathode 28l of the second triode 286. The anode 283 of the second triode 280 is coupled to the grid 211 of the first triode 215 by means of a condenser 281 and the anode 218 of the first triode 215 is coupled to the grid 282 of the second triode 286 by a condenser 288. The grid 211 of the first triode 215 is connected to the input terminal 21!. The anode 218 of the first triode 215 is connected to the first output terminal 212 and the anode 283 of the second triode 286 is connected to the second and third output terminals 213 and 214. The circuit constants of the elements of the multivibrator 216 are so selected that when oscillating freely it oscillates at a free frequency slightly less than half the power line frequency, that is, at a frequency slightly less than 30 cycles per second. Preferably the free period equals the duration of the first operating portion of the cycle of operation that is the free period preferably equals ,4, sec. plus the duration of one of the pulses generated by the impulse generator 236. In any event the free period is at least as long as the interval of the first operating portion of the operating cycle of the entire control system 219 'but substantially less than three quarters of a cycle of operation.

The multivibrator 216 is synchronized with the rotation of the magnetic field produced by the inductive field generator I56. This is accomplished by applying positive pulses from the impulse generator 236 to the grid 211 of the first triode 215 thus forcing the multivibrator to oscillate at half the power line frequency. When thus synchronized, a rectangular voltage wave is produced at the respective anodes 218 and 283 of the triodes 215 and 286. The rectangular voltage wave thus generated at the anode 218 of the first triode 215 is applied to the'first output terminal 212 and rises to a relativeLv high value during the first operating portion of the cycle of operation and a relatively low value during the second operating portion of the cycle of operation as previously explained and as illustrated in Fig. iii. The rectangular voltage wave thus generated at the anode 283 of the second triode 286 is applied to the second and third output terminals 213 and 214 and falls to a relatively low value during the first operating portion of the cycle of operation and rises to a relatively high value during the second operating portion of the cycle of operation as previously explained and as illuswas n 1 aeeagcau 302, a control grid 303. and an anode 305. The

anodes 300 and 305 of the two triodes 236 and BM are supplied with high positive potentials in a conventional manner through corresponding resistors 305 and 30!. The two cathodes 291 and 302 of the pentodes 296 and 30l are connected to ground through a common bias control resistor 308. The control grids298 and 303 of the two pentodes 296 and 30! respectively are connected to ground through corresponding first and second resistors309 and 3H! respectively. The control grid 288 of the first pentode 296 is coupled through the condenser 3 to the first input terminal 29! of the intensity control circuit 290, which terminal 29l is in turn connected to the third output terminal 214 of the multivibrator 210. The control grid 303 of the second pentode 311i is coupled through the condenser 3| 2 to the second input terminal 292 of the intensity control circuit 290, which terminal 292 is in turn connected to the second output terminal 233 of the impulse generator 230. The anode 305 of the second pentode 3! is coupled to the output terminal 293 of the intensity control circuit 290 through condenser 3l3 which in turn is connected to the control grid 202 of the cathode ray tube 200 as hereinabove explained.

Considering now the detailed operation of intensity control circuit 290, when rectangular voltage waves from the third output terminal 214 of the multivibrator 210 are impressed upon the control grid 298 of the first pentode 296, the current through the resistor 300 is varied in a similar manner so that a rectangular voltage wave is applied at the cathodes 291 and 302 of both pentodes 296 and 30I. The rectangular voltage wave thus applied to the pentodes 296 and 3M rises to a-relatively high positive value during the first operating portion of the cycle ofoperation and falls to a relatively low value during the second operating portion of the cycle of operation, the relatively high voltage preferably being sufiicient to bias the second pentode 30l beyond cutofi and the relatively low voltage being insufiicient to bias the second pentode 30l beyond cutoif. Also the negative pulses from the second output terminal 233 of the impulse generator 230 are impressed upon the control grid 303 of the second pentode 30l and'are of sufficient magnitude to bias the second pentode 30l beyondcutoff regardless of the value of the bias on the cathode 302 thereof. Thus the second pentode 30! is non-conductive only when the voltage wave impressed on the cathode 302 thereof is of a relatively high value or when a negative pulse is impressed on the control grid 303, thereof. As a result of the combined action of the electromotive forces thus supplied to the second pentode 30I from the multivibrator 210 and from the impulse generator 230, the voltage at the anode 305 of the second pentode 30I rises to a relatively high value during the first portion of the cycle of operation and for a short period at the time T3 in the middle of the second portion of the cycle of operation, and falls to a relatively low value-the rest of the time, the resultant voltage wave appearing at the anode 305 being illustrated in Fig. 110. The voltage wave thus generated at the anode 305 of the second pentode I electronic switch 320.

22 3M is applied through the coupling condenser M3 to the junction between the bias battery 2 and the resistor 2 l2 associated with the cathode ray tube 200, thus causing the negative bias on the intensity control grid 202 of the cathode ray tube 200 to be reduced to a relatively low value during the first portion of the cycle of operation and for a short period at the time T3.

Referring now more particularly to Fig. 9, there is illustrated a detailed wiring diagram of the This electronic switch includes first, second, third, and fourth amplifying pentodes 330, 340, 350, and 360, and first, second, third, and fourth bias control triodes 310, 380, 390, and 400, respectively associated therewith in cathode follower relation. The leading electromotive force produced at the second output terminal I16 of the first phase transforming network H4 is coupled to the control grid 33I of the first amplifying pentode 330 through a condenser 332 and a variable potentiometer 333, the slide wire 334 of the potentiometer 333 being directly connected to this grid 33I. The lagging electromotive force produced at the first output terminal I15 of the first phase trans: forming network I14 is coupled to the grid 3 of the second amplifying pentode 340 through a condenser 342 and a variable potentiometer 343, the slide wire 344 of the potentiometer 343 being directly connected to this grid 34!. The leading electromotive force produced at the second output terminal I89 of the second phase transforming network I8! is coupled. to the grid 35i of the third amplifying pentode 350 through a condenser 352 and a variable potentiometer 353, the slide wire 354 of this potentiometer 353 being directly connected to this grid 35l. The lagging electromotive force produced at the first output terminal I88 of the second phase transforming network I8! is coupled to the grid 36l of the fourth amplifying pentode 360 through a condenser 362 and a variable potentiometer 363. the slide wire 364 of the potentiometer 353 being directly connected to this grid 36 l. The cathode 335 of the first amplifying pentode 330 and the cathode 315 of the first bias control triode 310 are connected to ground through a common resistor 316. Similarly, the cathode 345 of the second amplifying pentode 340 and the cathode 385 of the second bias control triode 380 are connected to ground through a common resistor 386. Similarly, the cathode 355 of the third amplifying pentode 350 and the cathode 395 of the third bias control triode 390 are connected to ground through a common resistor 396. And similarlmthe cathode 365 of the fourth amplify ing pentode 360 and the cathode 405 of the fourth bias control triode 400 are connected to ground through a common resistor 406. The control grids 3H and 38l of the first and second bias control triodes 318 and 380 are connected to ground through a common resistor 3's! and are coupled through a common condenser 318 to first control terminal 321 which in turn is connected to the first output terminal 212 of the multivibrator 270. Likewise the control grids 39l and 40l of-the third and fourth bias control triodes 390 and 400 are connected to ground through a common resistor 39'! and are coupled through a common condenser 390 to second control terminal 328 which in turn is connected to the second output terminal 213 of the multivibrator 210.

Thus with this arrangement the rectangular voltage wave appearing at the first output ter- 

